Environmental concerns over the use of fossil fuels and their resource constraints have spurred increasing use of energy generated from renewable sources such as wind and solar. Despite the abundance and ready availability, power generated by solar and wind resources are variable and uncontrolled. An effective approach to smooth out the intermittency is to use electrical energy storage (EES), storing excessive energy and releasing it when needed. The EES technologies are also demanded to improve the reliability and security of future electrical grids. Among various EES technologies, electrochemical energy storage systems or batteries are capable of reversibly storing and releasing electrical energy without involving “Carnot” cycles. One such technology is based on a β″-Al2O3 solid electrolyte (BASE) and this type of electrochemical device is often referred as Na-beta batteries (NBBs).
The high round-trip efficiency, high energy density and capability of energy storage for a duration of hours has led to an increased interest in NBB technologies for renewable storage and utility applications, as well as for clean, efficient vehicles. However. NBBs are batteries typically constructed on thick tubular electrolytes and operate at relatively high temperatures. Thus, while having promise from a theoretical perspective, these NBBs have a number of disadvantages, such as high capital cost and performance/safety issues, that have limited market penetration of the technology,
One common cathode of NBBs is molten sulfur. This type of battery is known as sodium-sulfur (Na—S) battery. Na—S technologies for utility energy storage have also been developed. As an alternative to the molten S/Na2Sx cathode, porous Ni/NiCl2 structures impregnated with molten NaAlCl4 have been used in the so-called ZEBRA hatter.
The β″-Al2O3 solid electrolyte (BASE) belongs to beta-alumina group, which is characterized by structures composed of alternating closely-packed slabs and loosely-packed layers. The loosely-packed layers contain mobile sodium ions and are called conduction planes or slabs, in which the sodium ions are free to move under an electric field. The closely-packed slabs are layers of oxygen ions with aluminum ions sitting in both octahedral and tetrahedral interstices. These layers are referred as the spinel block, which is bonded to two adjacent spinet blocks via conduction planes or slabs. The sodium ions diffuse exclusively within the conduction layers perpendicular to the c axis. There are two distinct crystal structures in the group: β—Al2O3 (hexagonal; P63/mmc; αo=0.559 nm, co=2.261 nm) and β″—Al2O3 (rhombohedral; R3m; αo=0.560 nm, co=3.395 nm). The differ in chemical stoichiometry and stacking sequence of oxygen ions across the conduction layer and sodium-ion conductivity. β″—Al2O3 exhibits a higher sodium ionic conductivity (typically 0.2˜0.4 S cm−1 at 300° C.) and is the preferred phase for sodium battery electrolyte applications,
The tubular design has been the dominate geometry of NBBs since the inception. However, the tubular design has many drawbacks. First, a tubular design requires a relatively thick cathode and smaller active surface area for a given cell volume, which limits power and energy densities. Secondly, the thicker electrolyte (>1 mm) typically employed in tubular designs also limits the high power characteristics. The tubular design complicates the interconnect between single cells and also impacts the overall cell packing efficiency. Finally, for metal halide sodium batteries they are typically manufactured by providing the sodium on the cathode side as a salt, such as NaCl. The sodium is then separated from the chloride and transferred to the anode side during the charge/discharge process. This method of manufacture limits the charge density of the resultant battery. Also, during the charge/discharge cycle, sodium battery electrode compartment undergoes volumetric changes. These changes along with the vapor pressure of molten sodium metal and other gases can create a differential pressure within the cell. This also limits the charge density of the resulting battery, because too much pressure can rupture the electrolyte.
The present invention is designed to overcome these drawbacks.